1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system such as a copying machine or a printer which forms an image on a recording material by using the electrophotographic system, or an image forming apparatus of an electrostatic recording system.
2. Description of the Related Art
Today, there are available a greater number of electrophotographic image forming apparatus capable of forming color images. One among them is a color image forming apparatus of a so-called one drum intermediate transferring belt system which sequentially forms toner images of yellow, magenta, cyan and black on one photosensitive drum, sequentially superimposes and transfers the toner images on an intermediate transferring belt, transfers the toner images of the intermediate transferring belt en bloc onto a recording material, and forms color images by fixing unfixed toner images of the recording material thereon by heat. There is also a tandem type color image forming apparatus which arrays a plurality of photosensitive drums, forms toner images of yellow, magenta, cyan, black and the like on the photosensitive drums, transfers the toner images from the photosensitive drums to an intermediate transferring belt, transfers the toner images of the intermediate transferring images onto a recording material, and forms color images by fixing unfixed toner images of the recording material thereon by heat.
In these color image forming apparatus, in a secondary transfer process of transferring the toner images of plural colors formed on the intermediate transferring belt en bloc to the recording material, if a toner charge amount on the intermediate transferring belt varies from color to color, colors of low and high transferabilities are generated, causing a problem of a reduction in image quality.
To prevent the problem, a post charging device for setting toner to a predetermined charge amount on the photosensitive drum after development is disposed to face the photosensitive drum. Toner charge amounts of color toner images transferred to the intermediate transferring belt are made uniform, thereby preventing the aforementioned problem.
Meanwhile, toner density detecting means is disposed in the color image forming apparatus. This toner density detecting means detects a toner density of the toner image on the photosensitive drum, and plays the following two roles.
One of these roles is to detect how much a predetermined latent image is developed, and used for controlling developing efficiency of a developing device. In the case of 2-component development, a mixing ratio of toner and a carrier greatly affects developing efficiency. Thus, feedback is generally given for toner replenishment of the developing device. The other of the above roles is used for controlling a gradation of an output image. A plurality of latent images different in gradation are provided, toner images are formed, densities thereof are detected, and feedback is given for control factors concerning gradation control of a look-up table or the like.
Ordinarily, a toner image for density detection uses an image of a small patch-shaped area. Accordingly, a toner density detecting sensor is called a patch detecting sensor in some case. As it is an optical sensor, the toner density detecting sensor (referred to as “patch detecting sensor”, hereinafter) cannot correctly measure a toner density if a sensor surface is stained with toner.
To solve this problem, a proposal of disposing a shutter between a photosensitive drum and a patch detecting sensor is described in Japanese Patent Application Laid-Open No. H01-309082. There is also known a method of preventing sensor staining by disposing a cleaning member such as a brush on a backside of a shutter to collect toner stuck to a sensor surface.
According to the conventional configuration, however, problems described below are inherent. The patch detecting sensor is arranged below the developing device to detect a density of a toner image developed by the developing device. In consequence, the shutter must be opened when the density of the toner image is detected. If development is carried out while the shutter is open, toner scattered from the developing device is stuck to a detecting unit of the patch detecting sensor to stain it.
Thus, Japanese Patent Application Laid-Open No. 05-289463 describes a configuration in which a post charging device is arranged on a downstream side of a developing device with respect to a rotational direction of a photosensitive drum, a patch detecting sensor is arranged on a downstream side of the post charging device, and a transferring unit is arranged on a downstream side of the patch detecting sensor. Such a distance set between the developing device and the patch detecting sensor enables a reduction in staining.
Additionally, an air flow must be formed around the post charging device to remove ozone generated from the post charging device. For the purpose of saving space, Japanese Patent Application Laid-Open No. 2000-162852 describes a configuration in which an air flow for blowing toward a photosensitive drum and a flow of air sucked away from the photosensitive drum (a photosensitive drum side) are formed in a post charging device. With this configuration, ozone generated from the post charging device can be removed.
However, depending on an air flow in the post charging device, toner scattered from the developing device may be headed to the patch detecting sensor, causing a problem of sticking of the scattered toner to the patch detecting sensor even if the developing device and the patch detecting sensor are set apart from each other.